Re: [PATCH 4/5] strbuf_reset: don't write to slopbuf with ThreadSanitizer

On 15 August 2017 at 20:43, Junio C Hamano [off-list ref] wrote:

Martin Ågren [off-list ref] writes:

quoted

If two threads have one freshly initialized string buffer each and call
strbuf_reset on them at roughly the same time, both threads will be
writing a '\0' to strbuf_slopbuf. That is not a problem in practice
since it doesn't matter in which order the writes happen. But
ThreadSanitizer will consider this a race.

When compiling with GIT_THREAD_SANITIZER, avoid writing to
strbuf_slopbuf. Let's instead assert on the first byte of strbuf_slopbuf
being '\0', since it ensures the promised invariant of "buf[len] ==
'\0'". (Writing to strbuf_slopbuf is normally bad, but could become even
more bad if we stop covering it up in strbuf_reset.)

Signed-off-by: Martin Ågren <redacted>
---
 strbuf.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/strbuf.h b/strbuf.h
index e705b94db..295654d39 100644
--- a/strbuf.h
+++ b/strbuf.h

@@ -153,7 +153,19 @@ static inline void strbuf_setlen(struct strbuf *sb, size_t len)
 /**
  * Empty the buffer by setting the size of it to zero.
  */
+#ifdef GIT_THREAD_SANITIZER
+#define strbuf_reset(sb)                                             \
+     do {                                                            \
+             struct strbuf *_sb = sb;                                \
+             _sb->len = 0;                                           \
+             if (_sb->buf == strbuf_slopbuf)                         \
+                     assert(!strbuf_slopbuf[0]);                     \
+             else                                                    \
+                     _sb->buf[0] = '\0';                             \
+     } while (0)
+#else
 #define strbuf_reset(sb)  strbuf_setlen(sb, 0)
+#endif


 /**

The strbuf_slopbuf[] is a shared resource that is expected by
everybody to stay a holder of a NUL.  Even though it is defined as
"char [1]", it in spirit ought to be considered const.  And from
that point of view, your new definition that is conditionally used
only when sanitizer is in use _is_ the more correct one than the
current "we do not care if it is slopbuf, we are writing \0 so it
will be no-op anyway" code.

I wonder if we excessively call strbuf_reset() in the real code to
make your version unacceptably expensive?  If not, I somehow feel
that using this version unconditionally may be a better approach.

What happens when a caller calls "strbuf_setlen(&sb, 0)" on a strbuf
that happens to have nothing and whose buffer still points at the
slopbuf (instead of calling _reset())?  Shouldn't your patch fix
that function instead, i.e. something like the following without the
above?  Is that make things noticeably and measurably too expensive?

Good thinking. There are about 300 users of strbuf_reset and 10 users of
strbuf_setlen(., 0) with a literal zero. Obviously, there might be more
users which end up setting the length to 0 for some reason or other. So
your idea seems the better one. I would assume that whoever resets a
buffer is about to add something to it, which should be more expensive,
but that's obviously just hand-waving. I'll see if I can find some
interesting caller and/or performance numbers.

When writing my patch, I used assert() and figured that with tsan, we're
in some sort of "debug"-mode anyway. If we decide to always do the
check, would it make sense to do "else if (strbuf_slopbuf[0]) BUG(..);"
instead of the assert? Or, if we do prefer the assert, would the
performance-worry be moot?

Thanks for the feedback.